Tierra was the first work to demonstrate that computer programs were capable of meaningful and complex evolution [1]}. The primary innovation was in designing a mutationally robust programming language, Tierran, which gave careful consideration to the way in which the representation in a programming language affects its evolvability. Tierra demonstrated the possibility for the emergence of thousands of unique programs from a single ancestral organism. Parasites evolved, which were capable of exploiting the replication code of other organisms. These parasites were eventually exploited by hyper-parasites, which used the parasite's exploitation to replicate themselves in cycle of ever-evolving digital ecology. Avida extends work in Tierra under a set of different dynamics [2]}. Most notably, each program contains its own region of memory that is inaccessible to other programs, and executes its code with a separate virtual CPU. The second major difference is that organism CPU execution speed is determined by its metabolic success, measured in its ability to perform logical operations.
